1. Field of the Invention
The present invention generally relates to an optical pickup device and, more particularly, to an optical pickup device adapted to. handle two kinds of optical information recording media.
2. Description of the Related Art
Conventionally, a compact disc (hereinafter, referred to as CD-R) is known as an optical information recording medium. A digital videodisc (hereinafter referred to as DVD) having a higher recording density has been put into practice. The DVD has a thickness different from the thickness of the CD-R. Additionally, a frequency of a beam used for the DVD is different from that of the CD-R. Thus. a conventional optical pickup device for the CD-R cannot be used for recording information on or reproducing information from the DVD. Similarly, an optical pickup device designed for the DVD cannot be used for recording information on or reproducing information from the CD-R.
In order to enable an optical disc apparatus to record information on or reproduce information from both the CD-R and the DVD, both the optical pickup device for the CD-R and the optical pickup device for the DVD may be provided in the optical disc apparatus so that one of the optical pickup devices is selectively used in response to a disc to be used. However, this method increases size and cost of the optical disc apparatus since two separate optical pickup devices must be provided in the optical disc apparatus.
From this point of view, an attempt was made to share a part of an optical system of each of the optical pickup devices by providing two light sources emitting light beams having different wavelengths. In this case, if an objective lens is shared by the two optical pickup devices, the following problem may occur. That is, if an objective lens, which is designed to provide a desired performance for the DVD, is used for the CD-R, a wavefront aberration is deteriorated due to a difference between wavelength or thickness when the objective lens is used for the CD-R. The deterioration of the wavefront surface may cause side lobes in a beam spot, which results in a decrease in a peak strength and expansion of a spot diameter. Accordingly, an operation on the CD-R cannot be performed in a good condition.
In order to solve the above-mentioned problem, in Japanese Laid-Open Patent Application No.8-55363, a method is suggested for changing an object distance in response to a selected optical information recording medium, the object distance being related to a beam incident on an objective lens from a light source. That is, a parallel beam is incident on the objective lens from the light source when information is recorded on the DVD. On the other hand, a divergent beam is incident on the objective lens when the CD-R is selected. In such a method, an appropriate beam spot can be formed on a recording surface of an optical recording medium irrespective of a thickness of the optical recording medium.
However, the above-mentioned patent document does not refer to a specific structure of a condenser lens that changes a light beam to a parallel beam or a divergent beam. Additionally, there is no consideration as to the deterioration of a beam spot associated with an offset generated in an optical axis of an objective lens.
It is a very important issue for an optical pickup device to obtain a high optical efficiency when the optical pickup is used for the DVD or the CD-R. Additionally, it is a very important issue to control the deterioration of a beam spot associated with an offset of an optical axis of an objective lens within an allowable range.
It is a general object of the present invention to provide an improved and useful optical pickup device in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide an optical pickup apparatus which shares an objective lens for two different light sources so as to handle two kinds of optical information recording media and which has a high optical efficiency and decreases deterioration in a beam spot caused by an offset of an optical axis of an objective lens.
In order to achieve the above-mentioned objects, there is provided according to one aspect of the present invention an optical pickup device applicable to both a first optical recording medium and a second optical recording medium, the second optical recording medium having a thickness larger than a thickness of the first optical recording medium, the optical pickup device comprising:
a first light source used for the first optical recording medium, the first light source. emitting a first beam having a first wavelength;
a second light source used for the second optical recording medium, the second light source emitting a second beam having a second wavelength;
an objective lens converging the first beam and projecting the converged first beam onto the first optical recording medium, the objective lens also converging the second beam and projecting the converged second beam onto the second optical recording medium,
an optical path guiding unit guiding the first beam and the second beam to the same optical path directed to the objective lens;
a signal extracting unit for extracting signals from the first and second beams reflected by a selected one of the first optical recording medium and the second optical recording medium; and
a second coupling lens coupling the second beam emitted from the second light source, the second coupling lens alone having a wavefront aberration substantially proportional to a field angle thereof so that the second beam after passing through the second coupling lens has a wavefront different from a wavefront of the first beam.
In the above-mentioned invention, the first beam emitted by the first light source toward the objective lens can be either a parallel beam or a convergent beam. Since the objective lens is designed to match the first optical recording medium having a smaller thickness, the objective lens can focus the first beam onto the first optical recording medium with an appropriate beam spot. On the other hand, the second beam is changed by the second coupling lens so that the second beam has a wavefront different from a wavefront of the first beam. That is, for example, if the first beam is incident on the objective lens as a parallel beam, the second beam is incident on the objective lens as a divergent beam. If the first beam is incident on the objective lens as a divergent beam, the second beam is incident on the objective lens as a divergent beam having a stronger divergence.
Thus, according to the above-mentioned invention, an appropriate beam spot can be formed on both the first optical recording medium having a smaller thickness and the second optical recording medium having a larger thickness, by changing the second beam to a beam which can be appropriately focused on the second recording medium by a combination of the second coupling lens and the objective lens.
In the optical pickup device according to the present invention, the objective lens may have a performance optimum for at least one of a recording operation, a reproducing operation and an erasing operation performed on the first optical recording medium.
Accordingly, a recording operation, a reproducing operation, or an erasing operation can be appropriately performed with respect to both the first optical recording medium having a smaller thickness and the second optical recording medium having a larger thickness.
In one embodiment of the present invention, the second coupling lens may provide a predetermined divergence to the second beam. Additionally, the optical pickup device according to the present invention may further comprise a first coupling lens changing the first beam into a parallel beam.
The second coupling lens may be a single lens, and one of opposite surfaces of the second coupling lens may be an aspherical surface and the other may be one of a spherical surface and a flat surface. According to this structure, the second coupling lens can be easily fabricated since one side of the second coupling lens is a flat surface or a spherical surface. Especially if one side of the second coupling lens is a flat surface, there is no need to consider an eccentricity between opposite sides of the second coupling lens. Thus, the design of the second coupling lens is easy.
Alternatively. the second coupling lens may include two single lenses coupled to each other, each of opposite surfaces of the second coupling lens may be an aspherical surface, and a coupling surface between the opposite surfaces of the second coupling lens may also be an aspherical surface. According to this structure of the second coupling lens, an amount of a wavefront aberration is easily controlled, which results in an easy design of the second coupling lens.
Additionally, in the optical pickup device according to the present invention,
the second coupling lens may be a single lens having opposite surfaces of which one is a flat surface and the other is an aspherical surface; and
the second coupling lens may be designed to provide an optimum performance to a divergent beam having an object distance which satisfies a relationship lc=xcex1xc2x7fc, where lc is a designed object distance, fc is a focal length and xcex1 is a constant.
According to this invention, when the second coupling lens is designed in the same manner as a normal lens without using a special evaluation function, an object distance which is one of the design conditions can be easily estimated and an easy design of the second coupling lens can be achieved.
Additionally, the optical pickup device according to the present invention may further comprise:
a first returning beam receiving unit receiving a first returning beam reflected by the first optical recording medium and returning toward the first light source;
a first optical path separating unit separating an optical path of the first returning beam from an optical path of the first beam emitted by the first light source;
a second returning beam receiving unit receiving a second returning beam reflected by the second optical recording medium and returning toward the second light source; and
a second optical path separating unit separating an optical path of the second returning beam from an optical path of the second beam emitted by the second light source,
wherein the first light source, the first returning beam receiving unit and the first optical path separating unit together are integrated as a first light source/light receiving unit, and the second light source, the second returning beam receiving unit and the second optical path separating unit together are integrated as a second light source/light receiving unit.
In one embodiment, the first optical path separating unit may utilize a state of polarization of the first beam so as to separate the optical path, and the second optical path separating unit may utilize a state of polarization of the second beam so as to separate the optical path.
Additionally, the first optical recording medium may have a base plate having a thickness of 0.6 mm and the second optical recording medium may have a base plate having a thickness of 1.2 mm, and a magnification of an optical system comprising the second coupling lens and the objective lens may be more than 0.2.
Additionally, there is provided according to another aspect of the present invention an optical pickup device applicable to both a first optical recording medium and a second optical recording medium, the second optical recording medium having a thickness larger than a thickness of the first optical recording medium, the optical pickup device comprising:
a first light source used for the first optical recording medium,-the first light source emitting a first beam having a first wavelength;
a second light source used for the second optical recording medium, the second light source emitting a second beam having a second wavelength;
an objective lens converging the first beam and projecting the converged first beam onto the first optical recording medium, the objective lens also converging the second beam and projecting the converged second beam onto the second optical recording medium,
an optical path guiding unit guiding the first beam and the second beam to the same optical path directed to the objective lens;
a signal extracting unit for extracting signals from the first and second beams reflected by a selected one of the first optical recording medium and the second optical recording medium;
a first aperture limiting member limiting the first beam entering the objective lens; and
a second aperture limiting member limiting the second beam entering the objective lens,
wherein one of the first and second aperture limiting members includes a wavelength selective filter having a wavelength selectivity for transmitting one of the first and second beams and reflecting the other of the first and second beams so as to limit an opening area thereof.
Accordingly, a diameter of each of the first beam and the second beam can be changed by the first and second aperture limiting members so that the first beam has a diameter appropriate for forming a good beam spot on the first optical recording medium and the second beam has a diameter by which side lobes are not generated in a beam spot. Thus, the optical pickup device according to this invention can appropriately perform a recording operation, a reproducing operation or an erasing operation with respect to both the first optical recording medium and the second optical recording medium.
In one embodiment of the present invention, the wavelength selective filter may be formed on a transparent flat plate on a side of the first and second light sources. Additionally, the wavelength selective filter may be formed on a transparent flat plate slanting with respect to an optical axis of a beam incident on the transparent flat plate.
In the optical pickup device according to the present invention, an aperture formed by the wavelength selective filter may be an oval, and the transparent flat plate may be slanted in a direction of a major axis of the oval with respect to a plane perpendicular to the optical axis of the beam incident on the transparent flat plate, a slanting angle of the transparent flat plate being set so that a projection of the aperture of the wavelength selective filter on a plane perpendicular to the optical axis of the beam becomes substantially a circle.
Additionally, in the optical pickup device according to the present invention,
the objective lens may be designed to provided a performance optimum to the first optical recording medium; and
the wavelength selective filter may be included in the second aperture limiting member so as to transmit the first beam and reflect the second beam.
The optical pickup device according to the above-mentioned invention may further comprise:
a first coupling lens changing the first beam into a parallel beam; and
a second coupling lens changing the second beam into a divergent beam.
Additionally, the optical pickup device according to the present invention may further comprise:
a first returning beam receiving unit receiving a first returning beam reflected by the first optical recording medium and returning toward the first light source;
a first optical path separating unit separating an optical path of the first returning beam from an optical path of the first beam emitted by the first light source;
a second returning beam receiving unit receiving a second returning beam reflected by the second optical recording medium and returning toward the second light source; and
a second optical path separating unit separating an optical path of the second returning beam from an optical path of the second beam emitted by the second light source,
wherein the first light source, the first returning beam receiving unit and the first optical path separating unit together are integrated as a first light source/light receiving unit, and the second light source, the second returning beam receiving unit and the second optical path separating unit together are integrated as a second light source/light receiving unit.
Further, in the optical pickup device according to the present invention, the optical path guiding unit may include a prism which transmits the first beam and reflects the second beam.
In one embodiment according to the present invention,
the first optical recording medium may be a disc having a thickness of 0.6 mm;
the second optical recording medium may be a disc having a thickness of 1.2 mm;
the first wavelength may be 650 nm and the second wavelength may be 780 nm; and
a numerical aperture of the first aperture limiting member may be 0.6 and a numerical aperture of the second aperture limiting member may be 0.5.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.